A number of lipid-based materials such as liposomes have been used as biological carriers for many pharmaceutical and other biological applications, particularly to introduce drugs, radiotherapeutic agents, enzymes, viruses, transcriptional factors and other cellular vectors into a variety of cultured cell lines and animals. Clinical trials have demonstrated the effectiveness of liposome-mediated drug delivery for targeting liposome-entrapped drugs to specific tissues and specific cell types. See, for example, U.S. Pat. No. 5,264,618, which describes techniques for using lipid carriers, including the preparation of liposomes and pharmaceutical compositions and the use of such compositions in clinical situations. More recently, cationic lipids have been used to deliver genes to cells, allowing efficient uptake and expression of foreign genes. While the basic methodology for using lipid-mediated vectors is well established, however, improvements in the materials used in the methods, both in terms of biocompatibility and in terms of effectiveness of the delivery process, are still desirable.
In particular, lipid-mediated delivery of exogenous nucleic acids in vivo in humans and/or various commercially important animals will ultimately permit the prevention, amelioration or cure of many important diseases and the development of animals with commercially important characteristics. The exogenous genetic material, either DNA or RNA, may provide a functional gene which, when expressed, produces a protein lacking in the cell or produced in insufficient amounts, or may provide an antisense RNA or ribozyme to interfere with a cellular function, e.g. a virus or cancer.
Nucleic acids are generally large polyanionic molecules which, therefore, bind cationic lipids through charge interactions. Lipid carriers have been shown to enhance gene delivery in vitro and in vivo. It is believed that positive charges of the lipid/DNA complexes mediate binding to negatively charged cellular membranes. By complexing with lipid carriers, DNA is protected from nucleases, enhancing receptor-mediated uptake and expression in the tissues and cells of interest. Felgner et al., (1994) J. Biol. Chem. 269(4):2550-2561. Typically, cationic lipids are mixed with neutral lipids, which allows the formation of stable liposomes. The liposomes are then complexed with nucleic acid, i.e., DNA, RNA or combinations of the two. Alterations in the lipid formulation allow targeting of DNA to different tissues in vivo. PCT patent application numbers WO 96/40962 and WO 96/40963. For in vivo applications, it is also desirable that the lipid carriers be biodegradable by the host, particularly with respect to treatment regimens that involve repeat administrations.